This Nonprovisional application claims priority under 35 U.S.C. xc2xa7 119(a) on Patent Application No(s). 2002-338840 filed in Japan on Nov. 22, 2002, the entire contents of which are hereby incorporated by reference.
This invention relates to a process for preparing organohalosilanes, and more particularly, to a process for preparing organohalosilanes by industrial direct reaction that can increase the productivity of useful silane in the organohalosilane product.
In the industry, organohalosilanes are produced by catalytic reaction of a contact mass of metallic silicon and copper catalyst with organohalides, known as Rochow reaction. In the production of methylchlorosilane, for example, not only dimethyldichlorosilane is formed as the main product, but methyltrichlorosilane, trimethylchlorosilane, methyldichlorosilane and methylchlorodisilanes are also formed as by-products. It is important to raise the yield of dimethyldichlorosilane having the largest demand among these silanes and to increase the reaction rate.
In this reaction, methyltrichlorosilane forms in a large amount next to dimethyldichlorosilane. The ratio in production weight of methyltrichlorosilane (T) to dimethyldichlorosilane (D) is generally indicated by the index T/D. Lower values of T/D are desired. In order to increase both the yield of dimethyldichlorosilane and the reaction rate, a number of engineers have made studies on the catalyst and metallic silicon, the system, the process and operating conditions.
Production of methylchlorosilanes is industrially carried out using fluidized bed reactors. The optimum value of reaction temperature is generally about 300xc2x0 C. though it varies somewhat with the type of catalyst. Maintaining the reaction temperature at an optimum level is very important to establish a high reaction rate and a high yield of useful silane at the same time. If the reaction temperature is too high, the amount of by-products formed increases. Probable causes include an increased amount of hydrogen group-containing silane presumably resulting from decomposition of methyl chloride, and the deposition of carbon on the contact mass surface and concomitant degradation of the contact mass. On the other hand, if the reaction temperature is too low, the reaction rate becomes slower, and if extremely low, production of dimethyldichlorosilane is also retarded.
On abrupt temperature changes, active sites on the contact mass surface serving as heat generation sources are expected to undergo greater temperature changes than macroscopic temperature readings by a thermocouple. It is then believed that the yield of dimethyldichlorosilane is sensitive to temperature changes within the reactor even if the changes are small. It is thus preferred to minimize the fluctuation of temperature within the reactor throughout the period of organohalosilane production.
Production of methylchlorosilane continues over about ten days to several weeks. During the period, fresh metallic silicon powder and catalyst powder are continuously or discontinuously supplemented as the metallic silicon powder is consumed and as the metallic silicon powder and catalyst powder are carried away from the system along with the fluidizing gas. Then the reactivity of the contact mass does not remain constant during the period and changes at all times under the influence of the properties (particle size, particle size distribution, impurity concentration, etc.) of metallic silicon, the type of catalyst, the properties (particle size, particle size distribution, impurity concentration, etc.) of catalyst, catalyst concentration, impurity concentration and the like. Since the changing reactivity is accompanied by perpetually changing amounts of heat generation, the maintenance of the reaction temperature requires a sophisticated control system. For the safety of production, it is also very important to maintain the temperature within the reactor at a certain value so as to prevent any temperature rise within the reactor.
In the production of methylchlorosilane, a heating operation of heating the contact mass charged to nearly the reaction temperature and a cooling operation following the start of heat generation due to reaction by introduction of methyl chloride are necessary in order to maintain the temperature within the reactor at a certain value. Heating may be carried out by circulating a heat medium oil through a jacket surrounding the reactor and/or an inner coil disposed within the reactor, while direct heating by a heater is also acceptable. Cooling may be carried out by circulating a (cool) heat medium oil through a jacket surrounding the reactor and/or an inner coil disposed within the reactor, while air cooling is also acceptable.
The reaction temperature within the reactor may be maintained at a desired level by manipulating the quantity of cooling. The quantity of cooling may be changed by changing the temperature or flow rate of heat medium or a combination thereof. However, the former means is difficult to maintain a constant temperature because a long time is taken to change the temperature of heat medium oil being circulated, indicating a slow response of temperature control. Still worse, the response becomes delayed as the apparatus becomes of larger size. Also, if the temperature of heat medium is lowered in order to increase the quantity of cooling, the wall temperature of the heat transfer surface becomes lower. Then high-boiling products and metal halide condense thereon, and metallic silicon powder and catalyst powder deposit on such condensates, causing to lower the coefficient of heat transfer.
The latter provides effective temperature control as compared with the former. The latter intends to increase the quantity of cooling by increasing the amount of heat medium oil circulated to thereby increase the coefficient of heat transfer. However, the circulation amount of heat medium oil and the coefficient of heat transfer are not in simple proportion. For example, when the heat medium oil is circulated in laminar flow, the coefficient of heat transfer, which is in proportion to an approximate ⅓ power of a flow velocity, is in proportion to an approximate ⅓ power of the circulation amount. Then, the circulation amount of heat medium and the coefficient of heat transfer are related as shown in FIG. 1. In the low flow rate region, the coefficient of heat transfer is sensitive to changes of circulation amount, but in the high flow rate region, the coefficient of heat transfer changes a little with changes of circulation amount. Then, in order to increase the precision of temperature control, the system must be designed with greater upper limits on the circulation amount of heat medium oil so that the system is operated in the low flow rate region during steady operation. This necessitates an extra capacity cooling unit relative to the cooling capacity for steady operation, increasing the cost of installation.
An auxiliary method of stabilizing temperature control is disclosed in JP-B 4-59318 where an inert solid powder is added to a contact mass containing metallic silicon and copper catalyst for ameliorating the temperature control in the fluidized bed reaction zone. Since the inert solid powder occupies a certain volume within the reactor, the amount of reactants admitted per reactor volume is reduced, resulting in a lower productivity.
U.S. Pat. No. 3,133,109 proposes the use of a heat transfer coil which is disposed as temperature control means in the reaction chamber of a fluidized bed reactor. With only the coil, the control effect is insufficient. JP-A 9-194490 discloses a method for preparing organohalosilanes at a high selectivity and in high yields by setting the product of a density by a linear velocity of organohalide-containing gas feed within a selected range. This method requires complicated management.
As discussed above, the prior art methods have problems including poor temperature control (response or stability), increased installation costs, and low productivity.
An object of the present invention is to provide a method for preparing organohalosilanes while keeping a certain temperature within the reactor to maintain a high productivity of useful silane, whereby organohalosilanes are produced at a low cost.
It has been found that by manipulating the partial pressure of organohalide gas in a gas feed to a reactor for organohalosilane production, the temperature within the reactor is maintained at a substantially constant appropriate level during the organohalosilane-forming reaction. Specifically, by manipulating the partial pressure of organohalide gas as one reactant to change the amount of heat generation due to reaction, the temperature within the reactor can be held at an appropriate level with a very high precision. The productivity of useful silane is maintained high. As a result, organohalosilanes are produced at a low cost.
Accordingly, the present invention provides a process for preparing oganohalosilanes comprising the steps of charging a reactor with a contact mass containing metallic silicon particles and a copper catalyst, and introducing an organohalide-containing gas feed into the reactor to effect reaction to form organohalosilanes, wherein the partial pressure of organohalide gas in the gas feed is manipulated so as to keep a substantially constant temperature within the reactor.